Molecular sieve

Molecular sieve (or short molecular sieve ) is the functional description of natural and synthetic zeolites or other substances that have a high adsorption capacity for gases, vapors and solutes with certain molecular sizes. By a suitable choice of the molecular sieve, it is possible to separate molecules of different sizes. In addition to zeolites, there are also carbon molecular sieves ( engl. carbon molecular sieve or molecular sieving carbon). On one hand, this process requires a bit more expensive, on the other hand they can be specialized during pyrolysis for specific separation.

The molecular sieves have a large inner surface area ( 600-700 m2 / g) and have uniform pore diameters which are in the order of the diameter of the molecules.

In the art, the pore diameters are often given in Angstroms. The molecular sieve is zeolite A having a pore size of 3 Å = 0.3 nm in the pore can thus only get in molecules that have a smaller ( effective ) diameter 0.3 nm. With molecular sieves gases and solvents can dynamically dried, separated mixtures of straight-chain and branched alkanes or water softening system.

Zeolite molecular sieves absorb nitrogen, act in medical oxygen concentrators via pressure swing adsorption ( PSA). To the other hand can win nitrogen CMS - Carbo Molecular Sieves are used.

Commercially are molecular sieves in powdered, rod-like or bead form. The most common pore sizes are 3 Å, 4 Å, 5 Å and 10 Å. Rod and bead-like molecular sieves are also available with a moisture indicator to display the achievement of the capacity limit.

For drying, the most common solvent in the laboratory chemistry a molecular sieve 4 Å is suitable. The pore width of 4 Å is unsuitable for solvents with relatively small molecules (eg, methanol, ethanol, dichloromethane or acetonitrile ) as the solvent molecules penetrate even into the pores and it can crowd out water located again. Such substances used molecular sieves of pore size 3 Å.

Adsorptivities

Depending on the pore size of the adsorption capacity of the material varies:

  • 3A ( 3 Å pore size ): Adsorbed NH3, H2O, ( not C2H6 ), suitable for drying polar solvents.
  • 4A ( 4 Å pore size ): Adsorbed H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6, EtOH. Not adsorb C3H8 and higher hydrocarbons. Good for drying nonpolar solvents and gases.
  • 5A ( 5 Å pore size ): Adsorbed normal (linear) hydrocarbons to n- C4H10, alcohols to C4H9OH, mercaptans to C4H9SH. Adsorbed no iso- compounds and rings greater than C4.
  • 10X (8 Å pore size ): Adsorbed branched hydrocarbons and aromatics. Suitable for the drying of gases.
  • 13X (10 Å pore size ) Adsorbed di -n -butylamine ( not tri-n- butylamine ). Suitable for drying HMPA.

Regeneration

An advantage of the molecular sieves is that they can be regenerated almost as often by heating to 350-400 ° C. Care must be taken on a slow heating, since the molecular sieve is aging too fast or otherwise is completely destroyed. Depending on how strong the material is bonded to the molecular sieve, the highest temperature must be adjusted during the drying process.

Molecular sieves can be regenerated in a different manner. Is it, for example, in equilibrium with the external concentration and this is reduced, for example by reducing the pressure, the adsorbed molecules evaporate from again until a new equilibrium is established.

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